1. Field of the Invention
The present invention relates to emitters for use as ion sources in mass spectrometry. In particular, it relates to field desorption emitters and to a process for their manufacture.
2. Discussion of the Prior Art
Mass spectrometry, although a powerful analytical technique, has been hindered, until recently by its requirement for sample volatility. The need to evaporate the sample before ionization, and the ionization process itself, have meant that highly polar, non-volatile compounds could not be studied. Conventional electron impact mass spectrometry imparts up to 70 eV energy to the sample, resulting in many fragmentation peaks and few, if any, molecular ion peaks for relatively polar compounds. The newer and more gentle processes of chemical ionization and field ionization do produce molecular ion peaks for fairly polar compounds, but the evaporation process involved in these techniques still requires enough energy to decompose many polar substances. Unfortunately, many compounds of biological and biochemical importance, such as nucleotides, nucleosides, carbohydrates, steroids, and amino acids, are polar, and their molecular weight determinations by electron impact, chemical ionization and field ionization have been mostly unsuccessful. The development of field desorption mass spectrometry has given rise to techniques applicable to these compounds and has extended the usefulness of mass spectrometry to even the most polar samples.
Field desorption involves the use of a high electric field gradient to induce tunneling of an electron from a sample to the surface on which that sample is absorbed. After the sample molecule loses its electron, it is repelled by the positively charged surface on which it is located and is driven into the gas phase by coulombic repulsion. The ions thus created enters the mass spectrometer analyzer region, are separated according to their mass to charge ratios, and are detected in the usual fashion.
There are several advantages that field desorption has over other types of ion formation techniques for mass spectrometry, especially when dealing with polar, non-volatile samples. First of all, unlike electron impact or field emission, the sample is not vaporized by heating before it is introduced into the ionization region. Instead it is adsorbed directly onto the site where ionization occurs. Samples which thermally decompose, therefore, will produce molecular ion peaks when field desorption ionization is used even though no such peaks are observed when electron impact ionization or field ionization are used. The ionization process involved in field desorption ionization is more gentle than that involved in electron impact ionization or in chemical ionization. Much less energy is imparted to the sample than the 70 eV normally used in electron impact ionization or the tens of kilocalories per mole used in chemical ionization. Therefore, less fragmentation occurs and the intensity of the molecular ion peak is correspondingly increased. The molecular ion peak intensity is further increased by the decreased length of time the molecular ions spend in the ionization region, because less time exists for the molecular ion to fragment. Finally, the decrease in fragmentation peak intensities and the corresponding increase in molecular ion peak intensities reduces the number of peaks in the spectrum produced by field desorption ionization, simplifying the identification of the unknown sample.
In field desorption, the sample is usually adsorbed onto the emitter, by dipping the emitter into the sample or applying the sample directly to the emitter using a syringe. The emitter is heated and the sample evaporated, as an ion, from it under the influence of the electric fields in the region of the emitter. In designing the emitter, therefore, two factors should be considered: (1) the geometry of the emitter surface, and (2) the surface area of the emitter. The geometry of the emitter surface determines the electric field strength experienced by the absorbed sample. The smaller the surface radius of curvature the higher the electric field, so it is desirable to have an emitter surface with either sharp edges or sharp points where very high electric fields can be obtained. Also, since the surface is intended to hold the sample solution, it is desirable to have an emitter with as large a surface area as possible so that an adequate amount of sample can be adsorbed onto the emitter.
In order to meet the requirements of the sharp points, the original field desorption emitters were thin platinum wires (2.5 .mu.m in diameter), thin foils, or razor blades. However, these emitters did not have surface areas large enough to accommodate an adequate amount of sample. The next type of emitter used was a thin wire on which a number of organic polymer crystals or carbon microneedles had been grown. The growth of these needles greatly increased the amount of ion current. These emitters were initially produced at low temperatures using acetone to form a polymer, but an attempt made to find the best chemical substance for needle growth led to the use of benzonitrile at high temperatures to produce carbon microneedles. Greater mechanical strength was obtained when a tungsten wire used as the support on which the needles were generated. This process generates carbon microneedles which exhibit good mechanical strength (they can be used as many as ten times) and which are affected only slightly by the sample being desorbed or by the high electric field. These sources, however, are difficult to produce. The equipment required for needle growth is elaborate, involving a good vacuum system (10.sup.-.sup.5 Torr) and a high voltage power supply (12 KV). Furthermore, benzonitrile is toxic, and the growth process requires a substantial amount of time (about 8 hours) with only a 50% success rate.
Recently, Gol'denfel'd et al. have done some work on the electrochemical deposition of low vapor pressure metals such as zinc, copper and iron to form emitters having metal dendrites attached to a tungsten support. This work was reported in Instruments and Experimental Techniques, Vol. 16, No. 3 Part 2, pages 852-4, May-June 1973 . While these filaments have a developed surface, the dendrites are generally short and the relatively high vapor pressure of the material from which they are made reduces their usefulness as a field desorption source.
There is, therefore, the need for a field desorption emitter which can be produced by a simple, reliable process, free from the disadvantages referred to above.